(1) Field of the Invention
This invention relates to methods and apparatus for applying a coating solution such as SOG (Spin On Glass, also called a silica coating material), photoresist or polyimide resin to substrates such as semiconductor wafers, glass substrates for photomasks, glass substrates for liquid crystal displays or glass substrates for optical disks (hereinafter referred to simply as substrates or as wafers). More particularly, the invention relates to a technique of supplying a coating solution to the surface of each substrate to form a coating film in a desired thickness thereon.
(2) Description of the Related Art
A conventional coating solution applying method of the type noted above will be described with reference to FIG. 1. FIG. 1 shows a principal portion of a substrate spin coating apparatus.
The illustrated apparatus includes a suction type spin chuck 10 for suction-supporting and spinning a substrate or wafer W in a substantially horizontal posture, and a supply nozzle 30 disposed substantially over the center of spin for supplying a photoresist solution R, which is a coating solution, to a surface of wafer W.
In a conventional coating solution applying method utilizing the apparatus with the above construction, rotational frequency controls are carried out as shown in the time chart of FIG. 2, for example, to form a photoresist film in a desired thickness on the surface of wafer W.
First, the spin chuck 10 is driven by a motor, not shown, to spin the wafer W at a predetermined rotational frequency R1 (e.g. 900 rpm). At a point of time t.sub.S at which the spin stabilizes, photoresist solution R begins to be delivered at a substantially constant flow rate from the supply nozzle 30. Photoresist solution R continues to be supplied to a region around the spin center of wafer W. The supply of photoresist solution R is stopped at a point of time t.sub.E which is a predetermined time after the photoresist supply starting point t.sub.S. A quantity of photoresist solution R supplied is determined such that substantially the entire surface of wafer W is covered by the photoresist solution R at the supply stopping point of time t.sub.E. Then, the rotational frequency of the spin chuck 10 is increased from rotational frequency R1 to rotational frequency R2 (e.g. 3,000 rpm). This higher rotational frequency R2 is maintained for a predetermined time. Consequently, a superfluous part of photoresist solution R supplied to the surface of wafer W is dispelled, thereby forming a photoresist film in a desired thickness on the surface of wafer W.
In order to adjust the thickness of the photoresist film, as shown in dotted lines in FIG. 2, the rotational frequency R2 may be decreased to form a thick film, or may be increased to form a thin film. The above photoresist solution supplying method in which the photoresist solution supply is started and stopped while the wafer W is spinning is called "dynamic method" in this specification.
On the other hand, a supplying method different from the dynamic method may be employed in which, as shown in parentheses in FIG. 2, the photoresist solution supply is started at a point of time (t.sub.S) when the wafer W is stationary, and stopped after rotation of the wafer W begins. This supplying method is a combination of the above dynamic method and a supplying method (hereinafter called static method) in which the photoresist solution supply is started and stopped while the wafer W is maintained stationary. Thus, this supplying method will be called "stamic method" in this specification.
The conventional photoresist solution applying method noted above has the drawbacks set forth hereunder. While these drawbacks will be described, taking the "dynamic method" for example, they are equally applicable to the "stamic method".
After supply of the photoresist solution R is started at the point of time t.sub.S in FIG. 2, the photoresist solution R supplied to a region around the center of wafer W, while retaining a substantially circular shape in plan view, spreads concentrically under the centrifugal force due to the rotational frequency R1. Subsequently, however, the photoresist solution R supplied to the region around the center of wafer W does not spread evenly in radial directions. Instead, as shown in FIG. 3, from a circular drop Ra (hereinafter referred to as core Ra) of photoresist solution R present around the spin center of wafer W, the photoresist solution R begins to flow in a plurality of rivulets (hereinafter referred to as fingers Rb) extending radially toward the edge of wafer W.
As the photoresist solution continues to be supplied in this state, the fingers Rb first extend and reach the edge of wafer W. With a further supply of photoresist solution, the fingers Rb become broader while the core Ra increases in diameter. As a result, uncovered regions between the fingers Rb become filled with the photoresist solution R, until finally the entire surface of wafer W is covered by the photoresist solution R.
In the stage of rotational frequency R1, the entire surface of wafer W is covered by the photoresist solution R exhibiting the foregoing behavior. After the fingers Rb reach the edge of wafer W, a large part of the photoresist solution R on the wafer W flows through the fingers Rb to scatter to the ambient. In order to cover the entire surface of wafer W, it is necessary to take into account the quantity of photoresist solution R scattering to the ambient after flowing through the fingers Rb, and to supply the photoresist solution R in a correspondingly increased quantity. If the photoresist solution R were supplied in an insufficient quantity, the uncovered regions between the fingers Rb would not be filled with the photoresist solution R. Even if the rotational frequency of wafer W is switched to the rotational frequency R2, it is impossible to form a photoresist film of uniform thickness.
Thus, according to the conventional coating solution applying method, a large quantity of photoresist solution R must be supplied in order to form a photoresist film of uniform thickness on the wafer W. This poses a problem of a very low efficiency of using the photoresist solution R. The photoresist solution is far more expensive than a treating solution such as a developer or a rinse used in manufacture of semiconductor devices. Thus, to improve the efficiency of using the photoresist solution through a reduction in the quantity of scattering photoresist solution is an important consideration in achieving low manufacturing costs of semiconductor devices and the like.
In addition, there has been a trend recently to use larger semiconductor wafers in the manufacture of semiconductor devices. It is urgently required to achieve a technical objective of forming photoresist films of highly uniform thickness while cutting down the consumption of photoresist solution as much as possible. Attempts are being made today to adopt large wafers 300 mm in diameter rather than 8-inch (about 200 mm) wafers, for example. Processing of wafers having such a large diameter encounters the following inconvenience besides that noted above.
When a large wafer of 300 mm diameter is processed under the rotational frequency controls according to the time chart shown in FIG. 2, a photoresist film formed on the wafer surface has a drastically reduced uniformity in thickness. When a photoresist film is formed on the wafer W in a spin, turbulent gas flows are generated adjacent the edge of the wafer W. These gas flows are more conspicuous for large diameter wafers. The turbulence results in an uneven vaporization of solvent occurring with the photoresist solution spreading from the center to the edge of the wafer W. This phenomenon is believed to be the main cause of the drastically reduced uniformity in film thickness.
The rotational frequency of the wafer W must be lowered in order to suppress the turbulent gas flows to secure a uniform film thickness. Taking the foregoing controls for example, the rotational frequency R1 should be lowered from the conventional 900 rpm to 700 rpm, and the rotational frequency R2 from the conventional 3,000 rpm to 1,500 rpm. By lowering the rotational frequencies R1 and R2 to the stated levels, turbulent flows may be suppressed to improve the uniformity in film thickness. However, naturally, the photoresist solution will be dispelled from the wafer W in a reduced quantity, resulting in an increased thickness of photoresist film formed thereon. Thus, it is difficult to form a photoresist film of desired thickness, especially a thin photoresist film.
To obtain a photoresist film of desired thickness with a high degree of uniformity in film thickness, it has been proposed to adjust the viscosity (e.g. in units of 0.1 cp) of the photoresist solution according to a desired film thickness. That is, a thin film may be formed by supplying a photoresist solution of low viscosity. With such a technique, however, there is a limitation to the adjustable range of film thickness. After all, film thickness must be adjusted by varying the levels of rotational frequencies R1 and R2. However, in the case of a large wafer, the problem of non-uniform film thickness due to turbulent gas flows inhibits major adjustments to be made to the rotational frequencies R1 and R2 over the ranges that were possible in the past. Consequently, only a small range is allowed for adjusting the thickness of photoresist film while securing a high level of uniformity in film thickness.
The proposed technique of adjusting film thickness by varying the viscosity of the photoresist solution can hardly be said a realistic method of processing large wafers. The reason is that a solvent mixing mechanism is required for mixing a photoresist solution of fixed viscosity with a solvent in a quantity corresponding to a desired film thickness, or a photoresist solution switching mechanism for selecting a photoresist solution from photoresist solutions of varied viscosity levels prepared in advance, to be supplied according to a film thickness needed. In any case, the apparatus and its controls will be extremely complicated.